Hitherto, a polyvinyl alcohol film has been produced by dissolving a polyvinyl alcohol resin in a solvent such as water to prepare a stock solution, subsequently forming a film by a solution-casting method (hereinafter, referred to as a casting method), and drying the film using a metal heating roll or the like. The polyvinyl alcohol film thus obtained has been utilized in a large number of applications as a film excellent in transparency and dyeing property, and a polarizing film is cited as one of useful applications thereof. Such a polarizing film has been used as an elemental constituent element of liquid crystal displays and in recent years, its use has been extended to equipments for which high definition and high reliability are required.
Under such circumstances, as the screen size of a liquid crystal television set or the like is enlarged, there has been desired a polarizing film having smaller number of optical defects as compared with conventional ones. As measures therefor, it have been proposed to use a polyvinyl alcohol film containing optical foreign particles having a size of 5 μm or more in an amount of 500 or less particles per 100 cm2 (for example, see Patent Document 1) or to use a polyvinyl alcohol film containing optical foreign particles having a size of 20 μm or more present on the surface of the film in an amount of 10 or less particles per 1 m2 (for example, see Patent Document 2).
However, causes of the optical defects are not only micron-order foreign particles and not only the foreign particles on the film surface. Submicron-order foreign particles, aggregates of additives, or inhomogeneous regions derived from polymer structures present inside the film are also causes of the optical defects. This is because the polarized light used in a liquid crystal may reflect or interfere in visible light regions, i.e., submicron-size minute regions having a submicron wavelength (generally 370 to 700 nm) and having a refractive index different from that of polyvinyl alcohol resin. In the case where the polarized light reflects or interferes, transmittance of the polarized light is disturbed and thus a light scattering phenomenon may occur.
Usually, for the purpose of mainly improving the film-forming ability, various additives such as a plasticizer and a surfactant are mixed into a polyvinyl alcohol film. Among these additives, there are some additives which form aggregates having a size of several hundred nm during the process of film formation. The aggregates formed in the film have a refractive index different from that of the polyvinyl alcohol film and hence may be a cause of light scattering. Herein, the aggregates may include aggregates formed by various mechanisms, such as those formed by electrostatic aggregation of the additives themselves, those formed by aggregation of subcomponents of the additives, or those formed by aggregation of decomposition products of the additives formed by heat during the film formation. When the aggregates are as minute as about several tens nm and are homogeneously dispersed in the film, light scattering may not occur. However, since plurality of additives are usually used sometimes in an extremely minute amount (ppm order) in the polyvinyl alcohol film, it has been hitherto difficult to specify causes of light scattering.
Since light scattering remarkably decreases fineness of images, in consideration of fineness enhancement, luminance enhancement, and area enlargement required for recent displays, it has been desired to develop a technology for further improving the light scattering through specifying causes of the light scattering.
Patent Document 1: JP-A-2001-316492
Patent Document 2: JP-A-2004-20631